Abstract: Narrow gap welding of 50 mm thick NM450 and ZG30SiMn dissimilar steels was carried out with double wire GMAW. Angle of welding wire and welding mode were explored in simulated narrow gap groove. It was found that when angle of welding wire was 6° and welding mode of two welders was constant pressure, well-formed welded joints without macroscopic defects could be obtained. In thick plate welding, two arcs could burn stably on sidewall, and melt could flow sufficiently and wet sidewall to make weld appear concave. Due to presence of angle between two wires, incomplete fusion of sidewall was effectively suppressed, and no macroscopic cracks appeared under the condition of no preheating. Microstructure of weld mainly consisted of a large amount of acicular ferrite and a small amount of massive ferrite. There was more massive ferrite in covering layer, and massive ferrite was relatively large, while number of massive ferrite in backing layer and filling layer was small, and massive ferrite was relatively small. Consequently, microhardness of backing layer was greater than that of filling layer and microhardness of filling layer was greater than that of covering layer.